1. Field of the Invention
The present invention relates in general to a hot glue dispenser or “glue gun.” More specifically, the present invention is related to a hot glue dispenser and a voltage regulating circuit to control the heat produced by the element, but also has a power control circuit that allows the heating element to obtain fill heat and then pulses it to maintain the set heat which reduces the power consumption.
2. Description of Related Art
A hot glue dispenser, or gun, is an appliance for liquefying and dispensing thermoplastic materials such as hot melt adhesives and glues from their solid phase through the controlled application of heat energy. The liquefied glue, or “melt,” is then in a readily usable form for application to a workpiece. Typically, prior art glue guns are comprised of three major parts: the body; the electrical power circuit components, e.g., the heating element(s) and control(s); and the mechanical components, e.g., the glue path and application mechanism. The mechanical components associated with a typical glue gun are described in U.S. Pat. No. 4,523,705 issued to Belanger et al. entitled “Mechanism for Glue Gun,” which is incorporated herein by reference in its entirety. Normally, the type of glue selected is dictated by the particular type of glue gun used for dispensing the gun. One popular form of glue is the hot melt “glue stick” which is elongated cylindrically shaped thermoplastic material which is received in a portable hot melt glue gun at a similarly shaped receiving end in the glue path. Further along the glue path is the melt chamber which is surrounded by one or more electrical heating elements for communicating radiant heat energy to the melt chamber and thermoplastic material therein. A mechanical feed mechanism is typically provided within the body for applying pressure to the solid portion of the flexible thermoplastic rod for forcing it along the glue path, into the melt chamber and finally dispensing the melt through an exit nozzle onto, for instance, a workpiece. The glue may also be of the “cool” melt type which liquefies at a lower temperature than hot melt type glue. Hot melt adhesives are appropriate for high temperature glue guns that typically operate at approximately 380° F. (193° C.) and are used for most bonding applications, craft projects, floral arrangements, repairs, and on most materials such as paper, wood, plastics, etc. The melting point of a hot melt adhesive is lower than 380° F. to ensure that the glue is melted and dispensable from a 380° F. chamber. Cool melt adhesives, on the other hand, are appropriate for low temperature glue guns that typically operate at approximately 225° F. (107° C.) and are recommended for use on heat sensitive materials such as styrofoam, balloons, and fabrics. A typical prior art glue gun is permanently configured for either hot or cool melt applications, although some prior art glue guns have dual temperature switching capability.
The temperature of the glue in the melt chamber is determined by three basic factors: heat energy in, i.e., the amount and time heat is applied to the chamber; heat energy out, i.e., the heat loss of the gun, glue path and amount of glue dispensed; and heat energy absorbed, i.e., the thermal properties of the glue enabling it to change phases. A distinction should be understood between the temperature and heat energy. Temperature is a measure of the average amount of motion per molecule, or concentration of heat, while heat energy is a quantity of heat. Temperature is graduated in degrees and heat in calories such that one calorie is the quantity heat energy required to raise the temperature of one gram of water one degree C.
Prior art glue guns generally rely on a relatively uncomplicated electrical power circuit for regulating the heat of the glue, the most simple of which utilizes a fixed temperature heating element operating at a predetermined fixed temperature, say 380° F. (193° C.), which is electrically coupled to a power source. However, two heating elements may operate at the identical fixed temperature and still produce different levels of heat energy, e.g., by employing high capacity heating elements. For example, two glue guns with heating elements operating at the identical fix temperature of 380° F. might, but if one has a larger capacity heating element, it will be capable of liquefying more glue in the same time period.
Other prior art glue gun power circuits are only slightly more complicated than that described above. One exemplary power circuit used a thermostat, or other heat-sensing device, for regulating electrical power to the heating element. That power circuit consists of an optional switch connected between a power source and a thermostat, and a heating element electrically coupled between the thermostat and power source. With a thermostat protecting the glue from overheating, heating elements may be employed which operate at a much higher temperature. The combination of higher operating temperature and larger capacity elements enables higher volumes of glue to be emulsified and dispersed in a comparable time period.
Throughput, the volume of glue a gun is capable of liquefying and dispersing in a set time period is a continual problem plaguing glue gun manufacturers. Incorporating larger capacity heating elements in a gun increases throughput, but substantially increase the cost of the gun. Heating elements with higher operating temperatures are also more expensive and have the added disadvantage of reacting with thermoplastics that cannot tolerated a higher temperature in the melt chamber. This is especially true for colored adhesives.
One solution for increasing throughput is described by U.S. Pat. No. 4,059,204 issued to Duncan et al. entitled “System for Dispensing and Controlling the Temperature of Hot Melt Adhesive.” Duncan discloses a hot melt glue gun which includes an electronic circuit enabling the gun operator to set a desired temperature to which the glue is heated. The set point is automatically raised to a predetermined amount when the glue is flowing in order to compensate for the drop in temperature caused by the loss of heat to the glue and atmosphere.
Another solution for increasing throughput and lowering the glue gun costs is by incorporating a head sink in the melt chamber. The heat sink absorbs latent heat which would otherwise be exhausted to the ambient air when the melt chamber is at higher temperatures, for instance during idle periods when glue is not passing through the glue path, and then releases the heat energy back into the melt chamber when the glue passing through it is at a lower temperature than the sink, such as during active periods when large volumes of glue are passing through the glue path. Other improvements to the melt chamber are disclosed in U.S. Pat. No. 5,462,206 issued to Kwasie entitled “Melting Assembly for Thermoplastic Materials,” which is incorporated herein by reference in its entirety. While the heat sink increases throughput somewhat, with only a marginal increase in cost, additional operation costs can easily surpass initial savings in gun costs. This happens because with the addition of the sink, the surface area of the high temperature portion of the gun is also increased, thereby increasing the area of the gun exposed to the cooler ambient air, and increasing heat loss. Additionally, heat losses result from transferring the heat energy from the element to the sink, then to the chamber and, then finally to the glue which decreases the overall efficiency of the gun. Thus, while the sink may lower the initial glue gun cost through the use of smaller capacity and/or lower operating temperature heating elements, the guns are considerably inefficient.
Other prior art references describe increasing the efficiency of glue guns by identifying areas of high heat loss and insulating them, thereby increasing efficiency of heat energy transfer into and retained in the melt adhesive, such as U.S. Pat. No. 6,142,207 issued to Richardot entitled “Hot Melt Glue Applicator and Glue Stick for Use Therein,” which is incorporated herein by reference in its entirety. However, each prior art glue gun suffers from a paradox, attempting to increase throughput results in more expensive initial costs and substantially higher operating expenses, due to the inherent inefficiencies of the gun. While, setting for lower throughput results in higher lag times, which in turn increases operating expenses due to the operator's increased idle time.